si_penny So, I took my giffgaff sim out of my phone (Samsung Galaxy A5) and gave it a wipe, placed it back in and it still wasn't recognised. I tried another sim in my phone (an EE sim) and it recognised it without a problem. I tried my giffgaff sim in the phone which I took the EE sim out of and when it booted up it said that the SIM was missing. Therfore looks like it is an issue with my giffgaff sim, rather than my phone.
Does anyone have any ideas on:
1) Why this might have happened?
2) How I can remedy it and get my phone working again?
1) Like any piece of technology, SIMs fail sometimes.
2) It looks like you need to replace your SIM.
You can get a new SIM either by buying one from a local shop or supermarket (McColls, Co-Op, Sainsbury's to name a few) for about a pound, or by ordering one from giffgaff for free (though it can take up to 5 days to arrive), see here:
When you have the new SIM, you can do a SIM swap to transfer all your account details, including your phone number and any goodybags or airtime credit, to the new SIM. See this guide:
Make sure that you are logged into your account when you start, or you might accidentally create a new account, with a new phone number!